FIG. 1 shows a measurement system 100 attached to a fiber 102 comprising a plurality of gratings 104, 106, and 108. A broadband source 126 is applied via fiber 128 to splitter 130, which sends optical energy to sensor array 102, which is a single fiber having gratings 104, 106, through 108 present. There may be an arbitrary number of such gratings, and each grating responds to a unique wavelength λ1, λ2, through λn. The gratings 104, 106, through 108 return optical energy at a center wavelength specific to each grating, and these reflected optical signals are removed in splitter 130 to wavelength measurement device 132, which may be any system for discriminating a plurality of wavelengths. In the wavelength measurement system 132, the incoming wave is sent to a wavelength separator 236, which performs coarse separation of wavelength, and this is followed by wavelength discriminators 138, 146, 154, which have a sine characteristic for a given wavelength, splitting the output between detector 1 134 and detector 1′ 136 to controller 120 analog complementary inputs 160 and 162. This same system of complementary wavelength discriminators is used for each incoming wavelength channel, comprising discriminators 146 for channel 2 complementary detectors 142 and 144 coupled to controller inputs 164 and 166, and for channel n, wavelength discriminator 154 coupled to complementary detectors 150 and 152 driving controller inputs 168 and 170. For each grating wavelength, there is a corresponding input detector pair which uses power ratio to determine wavelength.
FIG. 2 shows the broadband source 126 of FIG. 1 controlled by a signal 180 causing the broadband source 126 of FIG. 1 to be commutated on and off. Each of the detector pairs responding to a sensor grating reflecting optical power at a wavelength produces an output and a complementary output, shown as signals 182 and 184 which may be the signal outputs of exemplar detectors 134 and 136, or any of the other detector pairs. These signals are summed in signal 188 and subtracted from each other in signal 186. The ratio of the sum and difference signal may be used to form a computation shown as signal 190 which may be applied to a lookup table, or any mathematical relationship which can be used to determine the wavelength of the reflected optical energy. In this manner, a plurality of optical sensors return a plurality of unique wavelengths which are applied to a particular set of detectors, the output being resolved by measurement device 132 of FIG. 1 by using power ratios, or any other suitable means for wavelength detection. During the time the optical source is off, the offsets are determined, and when it is on, the detector pair for each wavelength channel determines the actual wavelength from the difference divided by the sum separately for each detector pair forming a wavelength channel.
FIG. 3 shows another example of a wavelength detection system 220, comprising a plurality of tunable filters 242, 244, 246 and a single detector 240. The broadband source 234 couples optical energy through splitter 238 as before, and the plurality of sensors reflect this energy using gratings 224, 226, through 228. The reflected energy is directed through splitter 238 to a plurality of tunable filters 246, 244, 242 to detector 240. The tunable filters are controlled by signals 254, 252, 250 respectively, and the null point of the detector response to a tuned filter resolves the wavelength of the returned optical energy, which translates into the strain or temperature of an individual grating.
FIG. 4 shows the control voltages 280, 282, and 284 applied to the tunable filters 242, 244, 246. Each filter is swept in succession using ramp voltages 280, 282, 284, and the corresponding null points 292, 294, 296 determine the wavelengths of the associated gratings 224, 226, 228.
In the measurement system 100 of FIG. 1, or 220 of FIG. 3, the cost of the measurement system tends to be much larger than the cost of the fiber and serial gratings attached to them. Further, while it may be inexpensive to add each additional sensor grating to the measurement system, each sensor grating requires a corresponding wavelength discriminator comprising a wavelength separator port and complementary detectors for the system of FIG. 1. In each of these systems, the number of sensor gratings is matched by the number of wavelength resolution systems, either the detector pairs 160, 162 of FIG. 1, or the tunable filters 242 of FIG. 3.